Adds a scavenge GC pass to collect unmodified references

Adds a scavenge GC pass that collects unmodified references instead of
processing object groups. This mode can be controlled by setting
FLAG_scavenge_reclaim_unmodified_objects. By default this is turned off.
Also, modified a test case to suit the handle the new GC pass.

BUG=v8:4421
LOG=N

Review URL: https://codereview.chromium.org/1410593005

Cr-Commit-Position: refs/heads/master@{#31599}

include/v8.h

@@ -603,6 +603,13 @@ template <class T> class PersistentBase {
    */
   V8_INLINE void MarkPartiallyDependent();
 
+  /**
+   * Marks the reference to this object as active. The scavenge garbage
+   * collection should not reclaim the objects marked as active.
+   * This bit is cleared after the each garbage collection pass.
+   */
+  V8_INLINE void MarkActive();
+
   V8_INLINE bool IsIndependent() const;
 
   /** Checks if the handle holds the only reference to an object. */
@@ -5960,6 +5967,13 @@ class V8_EXPORT Isolate {
    */
   void VisitHandlesForPartialDependence(PersistentHandleVisitor* visitor);
 
+  /**
+   * Iterates through all the persistent handles in the current isolate's heap
+   * that have class_ids and are weak to be marked as inactive if there is no
+   * pending activity for the handle.
+   */
+  void VisitWeakHandles(PersistentHandleVisitor* visitor);
+
  private:
   template <class K, class V, class Traits>
   friend class PersistentValueMapBase;
@@ -7042,6 +7056,7 @@ class Internals {
   static const int kNodeStateIsNearDeathValue = 4;
   static const int kNodeIsIndependentShift = 3;
   static const int kNodeIsPartiallyDependentShift = 4;
+  static const int kNodeIsActiveShift = 4;
 
   static const int kJSObjectType = 0xb7;
   static const int kFirstNonstringType = 0x80;
@@ -7368,6 +7383,15 @@ void PersistentBase<T>::MarkPartiallyDependent() {
 }
 
 
+template <class T>
+void PersistentBase<T>::MarkActive() {
+  typedef internal::Internals I;
+  if (this->IsEmpty()) return;
+  I::UpdateNodeFlag(reinterpret_cast<internal::Object**>(this->val_), true,
+                    I::kNodeIsActiveShift);
+}
+
+
 template <class T>
 void PersistentBase<T>::SetWrapperClassId(uint16_t class_id) {
   typedef internal::Internals I;

src/api.cc

@@ -7593,6 +7593,15 @@ void Isolate::VisitHandlesForPartialDependence(
 }
 
 
+void Isolate::VisitWeakHandles(PersistentHandleVisitor* visitor) {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  i::DisallowHeapAllocation no_allocation;
+  VisitorAdapter visitor_adapter(visitor);
+  isolate->global_handles()->IterateWeakRootsInNewSpaceWithClassIds(
+      &visitor_adapter);
+}
+
+
 String::Utf8Value::Utf8Value(v8::Local<v8::Value> obj)
     : str_(NULL), length_(0) {
   if (obj.IsEmpty()) return;

src/flag-definitions.h

@@ -683,6 +683,8 @@ DEFINE_BOOL(verify_heap, false, "verify heap pointers before and after GC")
 #endif
 DEFINE_BOOL(move_object_start, false, "enable moving of object starts")
 DEFINE_BOOL(memory_reducer, true, "use memory reducer")
+DEFINE_BOOL(scavenge_reclaim_unmodified_objects, false,
+            "remove unmodified and unreferenced objects")
 
 // counters.cc
 DEFINE_INT(histogram_interval, 600000,

src/global-handles.cc

@@ -54,6 +54,8 @@ class GlobalHandles::Node {
                   Internals::kNodeIsIndependentShift);
     STATIC_ASSERT(static_cast<int>(IsPartiallyDependent::kShift) ==
                   Internals::kNodeIsPartiallyDependentShift);
+    STATIC_ASSERT(static_cast<int>(IsActive::kShift) ==
+                  Internals::kNodeIsActiveShift);
   }
 
 #ifdef ENABLE_HANDLE_ZAPPING
@@ -64,7 +66,11 @@ class GlobalHandles::Node {
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     index_ = 0;
     set_independent(false);
-    set_partially_dependent(false);
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      set_active(false);
+    } else {
+      set_partially_dependent(false);
+    }
     set_in_new_space_list(false);
     parameter_or_next_free_.next_free = NULL;
     weak_callback_ = NULL;
@@ -86,7 +92,11 @@ class GlobalHandles::Node {
     object_ = object;
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     set_independent(false);
-    set_partially_dependent(false);
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      set_active(false);
+    } else {
+      set_partially_dependent(false);
+    }
     set_state(NORMAL);
     parameter_or_next_free_.parameter = NULL;
     weak_callback_ = NULL;
@@ -106,7 +116,11 @@ class GlobalHandles::Node {
     object_ = reinterpret_cast<Object*>(kGlobalHandleZapValue);
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     set_independent(false);
-    set_partially_dependent(false);
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      set_active(false);
+    } else {
+      set_partially_dependent(false);
+    }
     weak_callback_ = NULL;
     DecreaseBlockUses();
   }
@@ -140,12 +154,23 @@ class GlobalHandles::Node {
   }
 
   bool is_partially_dependent() {
+    CHECK(!FLAG_scavenge_reclaim_unmodified_objects);
     return IsPartiallyDependent::decode(flags_);
   }
   void set_partially_dependent(bool v) {
+    CHECK(!FLAG_scavenge_reclaim_unmodified_objects);
     flags_ = IsPartiallyDependent::update(flags_, v);
   }
 
+  bool is_active() {
+    CHECK(FLAG_scavenge_reclaim_unmodified_objects);
+    return IsActive::decode(flags_);
+  }
+  void set_active(bool v) {
+    CHECK(FLAG_scavenge_reclaim_unmodified_objects);
+    flags_ = IsActive::update(flags_, v);
+  }
+
   bool is_in_new_space_list() {
     return IsInNewSpaceList::decode(flags_);
   }
@@ -349,6 +374,8 @@ class GlobalHandles::Node {
   // in_new_space_list) and a State.
   class NodeState : public BitField<State, 0, 3> {};
   class IsIndependent : public BitField<bool, 3, 1> {};
+  // The following two fields are mutually exclusive
+  class IsActive : public BitField<bool, 4, 1> {};
   class IsPartiallyDependent : public BitField<bool, 4, 1> {};
   class IsInNewSpaceList : public BitField<bool, 5, 1> {};
   class NodeWeaknessType : public BitField<WeaknessType, 6, 2> {};
@@ -646,10 +673,18 @@ void GlobalHandles::IdentifyWeakHandles(WeakSlotCallback f) {
 void GlobalHandles::IterateNewSpaceStrongAndDependentRoots(ObjectVisitor* v) {
   for (int i = 0; i < new_space_nodes_.length(); ++i) {
     Node* node = new_space_nodes_[i];
-    if (node->IsStrongRetainer() ||
-        (node->IsWeakRetainer() && !node->is_independent() &&
-         !node->is_partially_dependent())) {
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      if (node->IsStrongRetainer() ||
+          (node->IsWeakRetainer() && !node->is_independent() &&
+           node->is_active())) {
+        v->VisitPointer(node->location());
+      }
+    } else {
+      if (node->IsStrongRetainer() ||
+          (node->IsWeakRetainer() && !node->is_independent() &&
+           !node->is_partially_dependent())) {
         v->VisitPointer(node->location());
+      }
     }
   }
 }
@@ -687,6 +722,49 @@ void GlobalHandles::IterateNewSpaceWeakIndependentRoots(ObjectVisitor* v) {
 }
 
 
+void GlobalHandles::IdentifyWeakUnmodifiedObjects(
+    WeakSlotCallback is_unmodified) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    if (node->IsWeak() && !is_unmodified(node->location())) {
+      node->set_active(true);
+    }
+  }
+}
+
+
+void GlobalHandles::MarkNewSpaceWeakUnmodifiedObjectsPending(
+    WeakSlotCallbackWithHeap is_unscavenged) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    DCHECK(node->is_in_new_space_list());
+    if ((node->is_independent() || !node->is_active()) && node->IsWeak() &&
+        is_unscavenged(isolate_->heap(), node->location())) {
+      node->MarkPending();
+    }
+  }
+}
+
+
+void GlobalHandles::IterateNewSpaceWeakUnmodifiedRoots(ObjectVisitor* v) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    DCHECK(node->is_in_new_space_list());
+    if ((node->is_independent() || !node->is_active()) &&
+        node->IsWeakRetainer()) {
+      // Pending weak phantom handles die immediately. Everything else survives.
+      if (node->state() == Node::PENDING &&
+          node->weakness_type() != NORMAL_WEAK) {
+        node->CollectPhantomCallbackData(isolate(),
+                                         &pending_phantom_callbacks_);
+      } else {
+        v->VisitPointer(node->location());
+      }
+    }
+  }
+}
+
+
 bool GlobalHandles::IterateObjectGroups(ObjectVisitor* v,
                                         WeakSlotCallbackWithHeap can_skip) {
   ComputeObjectGroupsAndImplicitReferences();
@@ -757,13 +835,23 @@ int GlobalHandles::PostScavengeProcessing(
       // the freed_nodes.
       continue;
     }
-    // Skip dependent handles. Their weak callbacks might expect to be
+    // Skip dependent or unmodified handles. Their weak callbacks might expect
+    // to be
     // called between two global garbage collection callbacks which
     // are not called for minor collections.
-    if (!node->is_independent() && !node->is_partially_dependent()) {
-      continue;
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      if (!node->is_independent() && (node->is_active())) {
+        node->set_active(false);
+        continue;
+      }
+      node->set_active(false);
+    } else {
+      if (!node->is_independent() && !node->is_partially_dependent()) {
+        continue;
+      }
+      node->clear_partially_dependent();
     }
-    node->clear_partially_dependent();
+
     if (node->PostGarbageCollectionProcessing(isolate_)) {
       if (initial_post_gc_processing_count != post_gc_processing_count_) {
         // Weak callback triggered another GC and another round of
@@ -790,7 +878,11 @@ int GlobalHandles::PostMarkSweepProcessing(
       // the freed_nodes.
       continue;
     }
-    it.node()->clear_partially_dependent();
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      it.node()->set_active(false);
+    } else {
+      it.node()->clear_partially_dependent();
+    }
     if (it.node()->PostGarbageCollectionProcessing(isolate_)) {
       if (initial_post_gc_processing_count != post_gc_processing_count_) {
         // See the comment above.
@@ -955,6 +1047,16 @@ void GlobalHandles::IterateAllRootsInNewSpaceWithClassIds(ObjectVisitor* v) {
 }
 
 
+void GlobalHandles::IterateWeakRootsInNewSpaceWithClassIds(ObjectVisitor* v) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    if (node->has_wrapper_class_id() && node->IsWeak()) {
+      v->VisitEmbedderReference(node->location(), node->wrapper_class_id());
+    }
+  }
+}
+
+
 int GlobalHandles::NumberOfWeakHandles() {
   int count = 0;
   for (NodeIterator it(this); !it.done(); it.Advance()) {

src/global-handles.h

@@ -197,6 +197,10 @@ class GlobalHandles {
   // class ID.
   void IterateAllRootsInNewSpaceWithClassIds(ObjectVisitor* v);
 
+  // Iterate over all handles in the new space that are weak, unmodified
+  // and have class IDs
+  void IterateWeakRootsInNewSpaceWithClassIds(ObjectVisitor* v);
+
   // Iterates over all weak roots in heap.
   void IterateWeakRoots(ObjectVisitor* v);
 
@@ -204,7 +208,7 @@ class GlobalHandles {
   // them as pending.
   void IdentifyWeakHandles(WeakSlotCallback f);
 
-  // NOTE: Three ...NewSpace... functions below are used during
+  // NOTE: Five ...NewSpace... functions below are used during
   // scavenge collections and iterate over sets of handles that are
   // guaranteed to contain all handles holding new space objects (but
   // may also include old space objects).
@@ -220,6 +224,19 @@ class GlobalHandles {
   // See the note above.
   void IterateNewSpaceWeakIndependentRoots(ObjectVisitor* v);
 
+  // Finds weak independent or unmodified handles satisfying
+  // the callback predicate and marks them as pending. See the note above.
+  void MarkNewSpaceWeakUnmodifiedObjectsPending(
+      WeakSlotCallbackWithHeap is_unscavenged);
+
+  // Iterates over weak independent or unmodified handles.
+  // See the note above.
+  void IterateNewSpaceWeakUnmodifiedRoots(ObjectVisitor* v);
+
+  // Identify unmodified objects that are in weak state and marks them
+  // unmodified
+  void IdentifyWeakUnmodifiedObjects(WeakSlotCallback is_unmodified);
+
   // Iterate over objects in object groups that have at least one object
   // which requires visiting. The callback has to return true if objects
   // can be skipped and false otherwise.

src/heap/heap.cc

@@ -1501,6 +1501,22 @@ static bool IsUnscavengedHeapObject(Heap* heap, Object** p) {
 }
 
 
+static bool IsUnmodifiedHeapObject(Object** p) {
+  Object* object = *p;
+  DCHECK(object->IsHeapObject());
+  HeapObject* heap_object = HeapObject::cast(object);
+  if (!object->IsJSObject()) return false;
+  Object* obj_constructor = (JSObject::cast(object))->map()->GetConstructor();
+  if (!obj_constructor->IsJSFunction()) return false;
+  JSFunction* constructor = JSFunction::cast(obj_constructor);
+  if (constructor != nullptr &&
+      constructor->initial_map() == heap_object->map()) {
+    return true;
+  }
+  return false;
+}
+
+
 void Heap::ScavengeStoreBufferCallback(Heap* heap, MemoryChunk* page,
                                        StoreBufferEvent event) {
   heap->store_buffer_rebuilder_.Callback(page, event);
@@ -1619,6 +1635,12 @@ void Heap::Scavenge() {
   promotion_queue_.Initialize();
 
   ScavengeVisitor scavenge_visitor(this);
+
+  if (FLAG_scavenge_reclaim_unmodified_objects) {
+    isolate()->global_handles()->IdentifyWeakUnmodifiedObjects(
+        &IsUnmodifiedHeapObject);
+  }
+
   {
     // Copy roots.
     GCTracer::Scope gc_scope(tracer(), GCTracer::Scope::SCAVENGER_ROOTS);
@@ -1657,7 +1679,14 @@ void Heap::Scavenge() {
     new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
   }
 
-  {
+  if (FLAG_scavenge_reclaim_unmodified_objects) {
+    isolate()->global_handles()->MarkNewSpaceWeakUnmodifiedObjectsPending(
+        &IsUnscavengedHeapObject);
+
+    isolate()->global_handles()->IterateNewSpaceWeakUnmodifiedRoots(
+        &scavenge_visitor);
+    new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
+  } else {
     GCTracer::Scope gc_scope(tracer(),
                              GCTracer::Scope::SCAVENGER_OBJECT_GROUPS);
     while (isolate()->global_handles()->IterateObjectGroups(
@@ -1666,14 +1695,14 @@ void Heap::Scavenge() {
     }
     isolate()->global_handles()->RemoveObjectGroups();
     isolate()->global_handles()->RemoveImplicitRefGroups();
-  }
 
-  isolate()->global_handles()->IdentifyNewSpaceWeakIndependentHandles(
-      &IsUnscavengedHeapObject);
+    isolate()->global_handles()->IdentifyNewSpaceWeakIndependentHandles(
+        &IsUnscavengedHeapObject);
 
-  isolate()->global_handles()->IterateNewSpaceWeakIndependentRoots(
-      &scavenge_visitor);
-  new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
+    isolate()->global_handles()->IterateNewSpaceWeakIndependentRoots(
+        &scavenge_visitor);
+    new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
+  }
 
   UpdateNewSpaceReferencesInExternalStringTable(
       &UpdateNewSpaceReferenceInExternalStringTableEntry);